create a dimension called "Filter-Open Date Time Range" formula -> FormatDate([Open Date]; "HH")
Then Count the HH
Nightly Count -> =Count([Ref Num] Where([Filter-Open Date Time Range]="00" Or [Filter-Open Date Time Range]="01" Or [Filter-Open Date Time Range]="02" Or [Filter-Open Date Time Range]="03" Or [Filter-Open Date Time Range]="04" Or [Filter-Open Date Time Range]="05" Or [Filter-Open Date Time Range]="06" Or [Filter-Open Date Time Range]="19" Or [Filter-Open Date Time Range]="20" Or [Filter-Open Date Time Range]="21" Or [Filter-Open Date Time Range]="22" Or [Filter-Open Date Time Range]="23";[Filter-Open Date Time Range]);All)
I am using that Nightly Count in the report and time range is 7pm - 7 am.
below link tells you various ways of formatting a date field in CABI.
https://bobjblog.wordpress.com/2012/03/05/web-intelligence-date-formats/